The invention relates to the field of display devices for reproducing optical information. In particular, the invention relates to a display device which serves to visualize information by use of liquids as well as a corresponding method.
From the prior art there is known an almost overwhelming variety of display devices serving the reproduction of optical information, such as e.g. text, still images or even moving images (videos). While a large proportion of these display devices serves in particular to reproduce multi-colored and quickly varying information, solutions exist which focus on the reproduction of more static and optionally monochrome information.
As an example for such display devices, reference is made to the technology of “electronic paper” developed by the company E Ink (Cambridge, Mass./USA; www.eink.com). The document U.S. Pat. No. 6,120,588 (“Electronically addressable microencapsulated ink and display thereof”) is mentioned in this respect. Herein, a plurality of tiny globules is used which can be oriented in an electric field and which are, for example, colored in white and black in equal share. By specific application of a corresponding electric field to certain display areas, the areas appear to be colored, whereas other areas appear uncolored. To enhance contrast, an opposed electric field can be applied to the remaining areas. In particular advantageous is the fact that such devices can be operated with low power, however, they offer slow reaction times and moreover the disadvantage that only a few colors can be displayed.
The problem of high energy consumption is encountered, for example, by document U.S. Pat. No. 7,304,786 owned by the company Liquavista (NL). The display principle described herein is based on the use of colored liquids, more specific of a two phase mixture made up of colored liquid (oil) and a carrier liquid (water). Switching of a single image dot occurs by means of the effect of electrowetting. By means of this effect, the shape of the oil droplet incurring the color impression can be changed. However, the amount of colored liquid present in a respective image dot is constant and can only be modified with respect to its shape. By utilizing the effect of electrowetting, energy input can be dramatically lowered, since power needed therefore is comparatively low. However, the power must be provided always if an image deviating from a rest position is to be displayed. Such a rest position results from the fact that the colored liquid without power feeding is homogeneously spread over the area of an image dot resulting to a correspondingly fully colored image dot. Furthermore, such display devices are suitable only for operation at normal ambient temperature. Moreover, these pixels include merely one color, so that display of plural different colors can only be achieved by means of a close arrangement of differently colored “subpixels” and use of optical fusion thereof to perceive one pixel. Aging finally leads to undesired degradation after a while which cannot be remedied, since due to the fixed volume of colored liquid of an image dot no exchange of degraded liquid is possible.
A first disadvantage of the solutions proposed by the prior art is based on the fact that for displaying information an (electric) power must be present, even if the content of the display does not change. Another disadvantage of known display devices based on the use of colored liquid is seen in the fact that these display devices often include display elements (“pixel”) consisting of two chambers, one of them serving actually to display information, whereas the other one is used as a reservoir in case that the corresponding pixel is not to be displayed at the moment. These constructions are complex as well as error-prone. As further disadvantage is seen in the fact that such two chamber solutions comprise a correspondingly larger footprint, unless the chambers are arranged respectively below the corresponding image dot which in turn leads to fluidically complicated constructions. Solutions, which utilize pumps for moving the liquid(s) can barely be realized, since either a plurality of pumps were necessary or corresponding complex switching valves had to enable a selection of the pixel to be driven, resulting to in particular extremely long switching times for an image often comprised of many thousand of pixels.
US 2005/0072946 A1 discloses a fluidic display device having display elements comprising a cavity and wherein at least one liquid reservoir for providing at least one display liquid is provided. Moreover, each display element includes a dosing mechanism, consisting of valves, by means of which the display liquid can be dosed from said at least one liquid reservoir into said display element and vice versa.
DE 697 11 750 T2 discloses an electro capillary display sheet wherein by means of electrodes a liquid transport between a reservoir and a display pixel is induced and the display liquid is provided via a capillary tube.